A resin-sealed semiconductor device is known, in which a semiconductor element (semiconductor chip) is mounted over and fixed to a die pad (die stage) of a lead frame with die-attaching material (die-bonding material), in which electrode terminals of the semiconductor element and inner leads of the lead frame are connected to each other with bonding wires, and in which sealing resin covers the semiconductor element, the bonding wires, the die pad, and the inner leads.
In such a resin-sealed semiconductor device, various types of lead frames are applied depending on type and outside size of a semiconductor element, and the external connection terminal (outer lead) leading structure and arrangement in lead frames.
As an example of a lead frame, a lead frame having an average die-pad structure is known, which has a die pad of a plane size larger than the plane outside size of the semiconductor element to be mounted (for example, see Japanese Laid-open Patent Publication No. 04-134853).
On the other had, a lead frame having a small die-pad structure is also known, in which a die pad of a plane size smaller than the plane outside size of the mounted semiconductor element is supported by hanging leads (for example, see Japanese Laid-open Patent Publication No. 07-7124).
In a semiconductor device having a lead frame of the average die-pad structure, separation is likely to occur between the die pad and the die-attaching material, or between the die pad and the sealing resin, depending on the heat conditions for test or mounting.
It is said that the separation is caused by a decrease in the adherence between structural members based on material differences among the die pad, the die-attaching material, and the sealing resin, or based on the contact area between the die pad and the die-attaching material and based on the contact area between the die pad and the sealing resin.
On the other hand, with a lead frame having a structure in which a smaller die pad is employed (hereinafter, small die-pad structure), the smaller die pad provides a smaller contact area with the die-attaching material and a smaller contact area with the sealing resin, thereby increasing the contact area between the sealing resin and the semiconductor element, which has a relatively good adherence.
Therefore, the separation which would occur with the average die-pad structured lead frame is effectively prevented.
However, even with the small die-pad structure, cracks are likely to occur in the sealing resin of the semiconductor device, depending on the size of the small die pad and the resin thicknesses on the bottom surface thereof.
FIG. 10 and FIGS. 11A to 11C illustrate a semiconductor device 500 as an example of the resin-sealed semiconductor device having such a small die-pad structure.
In FIG. 10, the semiconductor device 500 is transparently viewed from the die pad (die stage) side, in which the sealing resin is partially removed for convenience. FIG. 11A is a cross-sectional view taken along the line X5-X5 in FIG. 10. FIG. 11B is a cross-sectional view taken along the line Y5-Y5 in FIG. 10. FIG. 11C is a cross-sectional view taken along the line Z5-Z5 in FIG. 10
In the semiconductor device 500, a semiconductor element (semiconductor chip) 31 is mounted over a die pad (die stage) 11 with die-attaching material (die-bonding material) 21. A plurality of electrode terminals 32 of the semiconductor element 31 are connected to inner leads 13a of lead terminals 13 through bonding wires 41. Four hanging leads 12 extend from the die pad 11.
The die pad 11, the semiconductor element 31, the bonding wires 41, the inner leads 13a of the lead terminals 13 and the hanging leads 12 are covered with sealing resin 51.
The outer leads 13b of the lead terminals 13 function as a terminal for external connection of the semiconductor device 500.
In the semiconductor device 500, each of the four hanging leads 12 has a relatively large width (width W7), extending from the die pad 11 orthogonally to each other.
In the semiconductor device 500, if the semiconductor element 31 is not mounted in parallel with the die pad 11, or if the die pad 11 becomes deformed (uneven) in resin-sealing process, the hanging leads 12 make contact with the semiconductor element 31. FIG. 11B illustrates such a state.
As indicated by the circle 61, the semiconductor element 31 is in contact with one of the hanging leads 12.
If such a contact portion appears, since each hanging lead 12 has a large width, the contact area between the hanging lead 12 and the semiconductor element 31 is relatively large. Near the contact portion, there is a portion which is left unfilled with the sealing resin 51.
In this case, the moisture contained in the sealing resin 51 concentrates in such an unfilled portion, and the adherence of the semiconductor element 31 and the hanging lead 12 with the sealing resin 51 therebetween decreases.
Therefore, separation occurs between the semiconductor element 31 and the hanging lead 12 near the contact portion if the semiconductor device 500 is left under high temperature conditions for remelting (reflowing) solder material in order to mount the semiconductor device 500 over a wiring board (motherboard) for an electronic device.
Specifically, since the semiconductor device 500 has been left under high temperature conditions, the moisture content therein expands, the poor contact between the semiconductor element 31 and the hanging lead 12 is further disrupted, thereby causing separation in a wide range. In FIG. 10, the portions 61 represent the separations.
If the solder material applied for mounting the semiconductor device 500 over a wiring board (motherboard) is Pb-free solder, which needs high temperature to remelt (reflow), the expanded moisture further expands the gap between the semiconductor element 31 and the hanging lead 12, thereby causing separation more easily in a wider range.
On the other hand, FIG. 12 and FIGS. 13A to 13C illustrate a semiconductor device 600 as an example of a resin-sealed semiconductor device having the small die-pad structure, together with a lead frame having narrow hanging leads.
In FIG. 12, the semiconductor device 600 is transparently viewed from the die pad (die stage) side, in which the sealing resin is partially removed for convenience. FIG. 13A is a cross-sectional view taken along the line X6-X6 in FIG. 12. FIG. 13B is a cross-sectional view taken along the line Y6-Y6 in FIG. 12. FIG. 13C is a cross-sectional view taken along the line Z6-Z6 in FIG. 12.
Also in the semiconductor device 600, a semiconductor element (semiconductor chip) 31 is mounted over and fixed to a die pad 11 of a lead frame with die-attaching material (die-bonding material) 21. A plurality of electrode terminals 32 of the semiconductor element 31 are connected to inner leads 13a of lead terminals 13 through bonding wires 41. Four hanging leads 12 extend from the die pad 11.
The die pad 11, the semiconductor element 31, the bonding wires 41, the inner leads 13a of the lead terminals 13, and the hanging leads 12 are covered with sealing resin 51.
The outer leads 13b of the lead terminals 13 function as a terminal for external connection of the semiconductor device 600.
In the semiconductor device 600, each of the four hanging leads 12 extending from the die pad 11 orthogonally to each other, has a width (width W8) smaller than the hanging leads 12 of the semiconductor device 500.
The narrow hanging lead 12 provides a small contact area with the semiconductor element 31. Even if there is any portion which is left unfilled with the sealing resin 51 near the contact portion, the volume of the portion is very small (not illustrated).
Therefore, even if the moisture content in the sealing resin 51 expands, the force expanding the gap between each hanging lead terminal 13 and the semiconductor element 31 is smaller, compared to the case of the semiconductor device 500.
Therefore, the intimate contact between the semiconductor element 31 and the sealing resin 51 is maintained near the contact interface, thereby preventing separation in a wide range.
However, in this structure, cracks are likely to occur in the sealing resin 51 on the margin of the die pad 11 after heat cycle test.
FIG. 13A illustrates such a state. In FIG. 13A, a portion 71 represents a crack which has occurred in the sealing resin 51.
When the semiconductor device 600 is heated or cooled, the sealing resin 51 is thermally expanded or shrank, which produces a thermal stress. A crack occurs by the thermal stress concentrating on the center of the semiconductor device.
When the thermal stress becomes as large as the shearing stress of the sealing resin 51, the crack 71 occurs in the sealing resin 51 from near the die pad 11 in the center of the semiconductor device 600.
The handing leads 12 of wider width contribute to the dispersion of the thermal stress. However, if the hanging lead 12 is narrow in width, the dispersion is not achieved effectively. In this case, it is difficult to prevent or reduce the concentration of the thermal stress.
As a result, the crack 71 is likely to occur in the sealing resin 51.
As has been described above, in the resin-sealed semiconductor device having the small die-pad structure, the separation between the sealing resin and each hanging lead, which is likely to occur in the average die-pad structure, is prevented or reduced.
However, even in the small die-pad structured semiconductor device, the thermal stress produced when a heat cycle test is conducted is not absorbed, so that a crack is likely to occur in the sealing resin near the die pad.